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Python’s interfaces for processing XML are grouped in the xml package.

Warning

The XML modules are not secure against erroneous or maliciously
constructed data. If you need to parse untrusted or unauthenticated data see
XML vulnerabilities.

It is important to note that modules in the xml package require that
there be at least one SAX-compliant XML parser available. The Expat parser is
included with Python, so the xml.parsers.expat module will always be
available.

The documentation for the xml.dom and xml.sax packages are the
definition of the Python bindings for the DOM and SAX interfaces.

The XML processing modules are not secure against maliciously constructed data.
An attacker can abuse vulnerabilities for e.g. denial of service attacks, to
access local files, to generate network connections to other machines, or
to or circumvent firewalls. The attacks on XML abuse unfamiliar features
like inline DTD (document type definition) with entities.

The Billion Laughs attack – also known as exponential entity expansion –
uses multiple levels of nested entities. Each entity refers to another entity
several times, the final entity definition contains a small string. Eventually
the small string is expanded to several gigabytes. The exponential expansion
consumes lots of CPU time, too.

quadratic blowup entity expansion

A quadratic blowup attack is similar to a Billion Laughs attack; it abuses
entity expansion, too. Instead of nested entities it repeats one large entity
with a couple of thousand chars over and over again. The attack isn’t as
efficient as the exponential case but it avoids triggering countermeasures of
parsers against heavily nested entities.

external entity expansion

Entity declarations can contain more than just text for replacement. They can
also point to external resources by public identifiers or system identifiers.
System identifiers are standard URIs or can refer to local files. The XML
parser retrieves the resource with e.g. HTTP or FTP requests and embeds the
content into the XML document.

DTD retrieval

Some XML libraries like Python’s mod:’xml.dom.pulldom’ retrieve document type
definitions from remote or local locations. The feature has similar
implications as the external entity expansion issue.

decompression bomb

The issue of decompression bombs (aka ZIP bomb) apply to all XML libraries
that can parse compressed XML stream like gzipped HTTP streams or LZMA-ed
files. For an attacker it can reduce the amount of transmitted data by three
magnitudes or more.

The documentation of defusedxml on PyPI has further information about
all known attack vectors with examples and references.

defusedxml is a pure Python package with modified subclasses of all stdlib
XML parsers that prevent any potentially malicious operation. The courses of
action are recommended for any server code that parses untrusted XML data. The
package also ships with example exploits and an extended documentation on more
XML exploits like xpath injection.

defusedexpat provides a modified libexpat and patched replacment
pyexpat extension module with countermeasures against entity expansion
DoS attacks. Defusedexpat still allows a sane and configurable amount of entity
expansions. The modifications will be merged into future releases of Python.

The workarounds and modifications are not included in patch releases as they
break backward compatibility. After all inline DTD and entity expansion are
well-definied XML features.